It is common to determine the characteristic impedance of a low-loss transmission line using the measured propagation constant through , assuming that the parallel conductance and that the parallel capacitance is constant. However, if the line is a coplanar waveguide or a microstrip, two dielectrics (substrate and air) are involved and its effective dielectric constant is frequency-dependent, which makes also frequency-dependent. If so, the assumption is inconsistent with causality constraints. This paper presents an attempt at determining by augmenting the above method in a way consistent with causality. A method of calibration comparison with 2nd-tier multiline TRL is used to extract and , following 1st-tier probe-tip calibration that is assumed to be valid at least up to 67 GHz. Then, a causal model, , of parallel admittance is built from GHz) by network synthesis. and are then used to determine including GHz. The method is applied to a CMOS transmission line from 20 MHz to 220 GHz. A well-built model gives well-behaved and plausible estimate of causal over the entire measurement frequency range.